1. Field of the Invention
Aspects of the present invention relate to a heated chemical treatment of wastewater treatment plant (WWTP) excess aerobic sludge, including aerobically digested sludge, waste sludge from oxidation ditches and other long-sludge residence time (SRT) activated sludge wastewater treatment processes, and waste sludge from conventional SRT activated sludge processes.
2. Description of the Related Art
Municipal WWTP sludge is typically composed of a combination of raw primary sludge and excess or waste activated sludge that is digested, either anaerobically or aerobically, to achieve solids mass reduction, vector attraction reduction, and a reduction in microbial indicators of fecal contamination such as fecal coliforms. In most cases, the digested sludge is subjected to mechanical dewatering to produce a drier material that can be incinerated, disposed of in a sanitary landfill, or applied in bulk to agricultural land as biosolids. Some producers of biosolids further dry the processed material to the point where it can be bagged and sold as a commercial soil conditioner/fertilizer (e.g., Milorganite produced by the Milwaukee Metropolitan Sewerage District).
WWTP sludge is generally processed to levels where it can meet Federal Class B sludge regulations. The Class B regulations represent the minimum levels of pathogen reduction that are acceptable for land application of biosolids (i.e., treated WWTP sludge). These regulations specify that wastewater sludge must be treated by a process to significantly reduce pathogens (PSRP) that will achieve a vector attraction reduction (VAR) goal of 38% reduction in volatile suspended solids (VSS) or meet a fecal coliform level in the processed sludge ≤2,000,000 MPN (Most Probable Number)/g, or alternately ≤2,000,000 CFU (Colony Forming Units)/g, based on the geometric mean of seven samples. Some states require municipal WWTPs to meet both stipulations to achieve a Class B rating. PSRPs include, among others, anaerobic sludge digestion at a mean cell residence time (MCRT) of at least 15 days at a temperature of 35° C.-55° C. and aerobic sludge digestion at a MCRT of at least 40 days at 20° C.
Land application of Class B biosolids, although widely practiced in the United States, has been accompanied by numerous and ongoing public complaints over the years. These complaints range from emanation of malodors from the applied fields to claims of illnesses and even deaths caused by volatilization of harmful compounds contained in the biosolids or direct contact with the biosolids. These complaints can be circumvented and most likely dispelled by the land application of biosolids treated to a higher level, namely Class A biosolids. The definition of Class A biosolids mandates the reduction of fecal coliforms and/or Salmonella to non-detect levels.
Prior research was conducted on anaerobically digested sludge produced on site in short-term 5-day MCRT bench-scale digesters at the University of Cincinnati (UC) (Cacho Rivero, 2005). Feed to the anaerobic digesters consisted of a mixture of primary and waste activated sludges from municipal WWTPs. The effluent sludge from these digesters was treated in a thermo-oxidation process in separate heated reactors. Hydrogen peroxide (H2O2) was added at doses ranging from 0.1-0.5 g/g volatile suspended solids (VSS) (dry wt.) and temperatures ranging from 35° C.-90° C. The higher doses and temperatures produced the greatest reduction in VSS. For example, at 90° C., VSS reductions of 58%, 65%, and 73% were achieved at H2O2 doses of 0.1, 0.25, and 0.5 g/g VSS, respectively. All of these VSS reduction levels are substantially greater than the minimum 38% reduction required for Class B sludge. The H2O2 dose was bled into the reactor over 6 hours to minimize foaming. The pH of the thermo-oxidation sludge remained largely unchanged, tending to increase slightly. At 90° C., no fecal coliforms were detected in the H2O2-treated sludge, thereby meeting the criteria for Class A biosolids.
Historically, WWTP design has utilized a two-stage treatment system configuration with a first-stage primary settling process followed by a second-stage biological treatment process. In the past, most WWTPs have utilized conventional activated sludge designs with SRTs in the range of 3-8 days as the second stage. Recently, particularly for WWTPs with low to moderate hydraulic capacity (i.e., 1-20 million gallons per day [mgd]), design engineers have determined it is more cost effective to eliminate first-stage primary settling of influent wastewater. Rather, influent wastewater is fed directly to a longer-SRT (>15 days) extended aeration activated sludge reactor, thereby obviating the need and cost of handling combined primary and waste activated sludges. Eliminating primary clarification in the treatment train and further because activated sludge reactors produce only aerobic sludge, there is less incentive to incorporate anaerobic digestion in the sludge treatment flowsheet.
Based on the above evolution in WWTP design philosophy, emphasis has shifted to the development of cost-effective methods for treating excess sludge from aerobic systems. It was postulated that the above thermo-oxidation concept would also perform well on excess activated sludge to produce Class A biosolids.
The theory behind the mating of first-stage biological treatment with follow-on second stage thermo-oxidation (chemical) treatment is to use the microorganisms in the biological treatment stage to cost-effectively oxidize most of the easy-to-degrade organics contained in the sludge matrix and to use the more expensive chemical (H2O2) treatment to oxidize the more recalcitrant organic compounds that are not easily degraded biologically. This treatment sequence optimizes what the biological and chemical stages do best and most efficiently. Highly oxidized excess sludge from WWTPs, whether produced in an aerobic digester or as mixed liquor sludge in an extended aeration activated sludge plant, and possibly even mixed liquor in a less oxidized conventional activated sludge process, are suitable for direct feed into the thermo-oxidation reactor. The thermo-oxidation process should be able to accommodate most sludges typically produced by municipal WWTPs.
Another benefit of the thermo-oxidation process is that some fraction of the nitrogen (particularly ammonia) inventory in the H2O2 feed sludge is solubilized during treatment in the thermo-oxidation reactor and can be recycled to the head of the treatment plant works in the reactor supernatant. If this did not happen, the entire nitrogen load would be transported to the application field in the biosolids. A significant fraction of this load, particularly the easily released ammonia component, would be rapidly solubilized and discharged into the soil, potentially exceeding the sorption capacity of the soil and contaminating ground water resources. By removing the easily released nutrient components in the WWTP sludge, the nutrients more tightly bound to the biosolids will be released slowly as needed for soil conditioning and fertilization.